The use of protective coatings on metal substrates for improved corrosion resistance and paint adhesion is common. Conventional techniques for coating such substrates include techniques that involve pretreating the metal substrate with a phosphate conversion coating and chrome-containing rinses. Typical phosphate conversion coatings operate in the range of about at least 1,000 parts per million (“ppm”) of phosphate, which leads to waste treatment issues. The use of such phosphate and/or chromate-containing compositions, therefore, imparts environmental and health concerns.
As a result, chromate-free and/or phosphate-free pretreatment compositions have been developed. Such compositions are generally based on chemical mixtures that in some way react with the substrate surface and bind to it to form a protective layer. For example, pretreatment compositions based on a group IIIB or IVB metal compound have recently become more prevalent.
When processing ferrous metal substrates through a pretreatment composition based on a group IIIB or IVB metal compound, however, the concentration of ferric (Fe+3) iron in a bath of the pretreatment composition increases over time as more iron based metal is treated. In particular, soluble (Fe+2) iron from the substrate becomes insoluble (Fe+3) through concentration build up and oxidation. The resulting insoluble rust, i.e., hydrated iron (III) oxide (Fe2O3.nH2O) and/or iron (III) oxide-hydroxide (FeO(OH)), (Fe+3) can deposit on the substrate and be carried to subsequent processing steps (particularly when filtration equipment is not available), such as a downstream electrocoat bath that is employed to deposit an organic coating. Such cross-contamination can detrimentally affect the performance of such subsequently electrodeposited coatings.
As a result, it would be desirable to provide improved methods for treating a ferrous metal substrate that addresses at least some of the foregoing.